1. Field of the Invention
The present invention relates to a method for coding and decoding information, especially for digital transmission or storage.
2. Description of the Prior Art
Source signals or source information such as voice, sound, image and video signals almost always contain statistical redundancy, that is to say redundant information. This redundancy can be greatly reduced by source encoding, making it possible to transmit and/or store the source signal efficiently. This redundancy reduction eliminates redundant signal contents which are based on the prior knowledge of, for example, statistical parameters of the signal variation, before the transmission. After the transmission, these components are added to the signal again in the source decoding, so that no loss of quality can be detected objectively.
Due to the incomplete knowledge of the source signals or restrictions in the complexity of the source encoding method, the source encoding usually only can be implemented in a suboptimum way, that is to say the compressed data still contain a certain redundancy even after the source encoding. In previous methods for source encoding, the source signals are frequently compressed to form symbols or quantized parameters which are then mapped to binary code words in accordance with an allocation rule, the allocation rule having been selected more or less randomly until now.
On the other hand, it is usual to deliberately add redundancy again by channel encoding during the signal transmission in order to largely eliminate the effect of co-channel interference on the transmission. Additional redundant bits thus enable the receiver or, respectively, decoder to detect and possibly also to correct errors.
It has long been one of the basic premises of information theory that source encoding and channel encoding can be carried out independently of one another in order to achieve an optimum result. According to this principle, the design of the source decoder depends only on the source characteristics whereas the channel encoding arrangement should depend only on the channel characteristics. This principle can be correct if the source encoder delivers statistically independent and thus uncorrelated symbols of equal probability and the decoding delay can be of any magnitude. In practical applications, however, these prerequisites are not met, as a rule. The output signal of the source encoder or, respectively, the symbol sequences output by it frequently exhibit a residual redundancy and, at the same time, the permissible delay is restricted, especially in voice transmission.
It is known to utilize this residual redundancy of the source-encoded symbol sequences in the so-called source-controlled channel decoding. In this process, the decoding process of the channel decoder is controlled, on the one hand, by the transmitted bits and, on the other hand, by a priori/a posteriori information on the most probable value of some important source bits. The source information thus has an influence on the result of the channel decoding. In the case of the Viterbi algorithm decoding, this method is called an a priori Viterbi algorithm. When such a method is used, only the receiver needs to be modified. Thus, the printed document J. Hagenauer, “Source-controlled channel decoding”, IEEE Trans. Commun., Vol. 43, pages 2449-2457, September 1995, teaches to use the inter-frame correlation, that is to say the statistical dependence between temporally and/or spatially adjacent signal samples in the source-controlled channel decoding.
Investigations have also shown that there is residual redundancy not only between bits of successive frames but also between the bits of a parameter within a frame due to the uneven distribution of the parameter values which, in turn, is attributable to the non-stationary condition of the source signals.